1. Field of the Invention
The invention relates to display devices, and in particular to transflective electrowetting display devices.
2. Description of the Related Art
Electrowetting display devices are rendered images in accordance with electrowetting or electrocapillary. Briefly, the free surface energy of some fluids is changed due to electric field effects such that distribution area of the fluids can thus change along with the electric field effects.
U.S. Pat. No. 6,967,763, the entirety of which is hereby incorporated by reference, discloses an electrowetting display device. An opaque non-polar fluid in the electrowetting display device converges due to electrowetting effect, thereby controlling a bright state and/or a dark state of a pixel. FIGS. 1A and 1B are cross sections respectively illustrating a voltage on-state and a voltage off-state for a conventional electrowetting display. Referring to FIG. 1A, a conventional electrowetting display 10 includes a substrate 16 with a patterned pixel electrode 15 disposed thereon. A dielectric layer 14 having a hydrophobic surface is disposed on the patterned pixel electrode 15. Patterned hydrophilic bank structures 13 are disposed on the dielectric layer 14, thereby defining a pixel region. An opaque non-polar fluid 12 containing a black dye and transparent polar fluid 11 are disposed in each pixel region. When the applied voltage is “off”, the opaque non-polar fluid 12 uniformly distributes in a pixel region, thereby rendering the pixel region to display a dark state.
On the contrary, when the applied voltage is “on”, the opaque non-polar fluid 12 is affected by electrowetting force and converged far away from the pixel electrodes 15. A large portion of the pixel region is thus exposed, thereby rendering the pixel region to display a bright state, as shown in FIG. 1B.
Conventional transflective color electrowetting displays use black oils as a light absorber and incorporatedly use color filters to achieve a full-color display. More specifically, incident light is passed through the electrowetting display and is reflected by a reflector or back light and directly passed through the electrowetting display to reach and pass through color filters to render full color images. The reflective regions and transmission regions of the conventional transflective color electrowetting display, however, are improperly arranged such that gray scale of the electrowetting display becomes difficult to control. Therefore, the stability and quality of electrowetting displays are affected by gray scale variations.
WO 2006/017129, the entirety of which is hereby incorporated by reference, discloses a transflective color electrowetting display structure in which a lower substrate and an upper substrate attached with color filters are assembled. A transparent polar fluid and a black non-polar fluid are interposed between the lower and upper substrate. The transflective color electrowetting display includes a plurality of pixels. Each pixel is divided into a transmission region and a reflective region on the lower substrate.
FIGS. 2A and 2B are cross sections respectively illustrating a voltage on-state and a voltage off-state of another conventional transflective electrowetting display. Referring to FIG. 2A, a conventional transflective electrowetting display 20 includes a first substrate 21 and a second substrate 29 opposing each other with a transparent polar fluid layer 23 and an opaque non-polar fluid layer 24 interposed therebetween. A first transparent electrode 22 is disposed on the first substrate 21. A second transparent electrode 27 is disposed on the second substrate 29. A dielectric layer 26 having a hydrophobic surface is disposed on the second transparent electrode 27. A reflector 28 is disposed under the second transparent electrode 27, thereby defining a reflective region and a transmission region. A backlight unit 35 is disposed on the back of the second substrate 29. A power supply 30 applies a bias between the first transparent electrode 22 and the second transparent electrode 27. Electrowetting force due to the bias causes convergence of the opaque non-polar fluid layer, thereby controlling reflective and transmissive regions of each pixel operation. When the applied voltage exceeds the saturated voltage, the opaque non-polar fluid layer 24 completely converges. Both the reflective and transmissive regions are entirely exposed, as shown in FIG. 2A.
When the applied voltage is greater than the threshold voltage but less than the saturated voltage, the opaque non-polar fluid layer 24 partially converges such that the exposed reflective area is greater than the exposed transmission area. Variations of the exposed reflective and transmissive regions can cause deviation of the gray scale of the electrowetting display, as shown in FIG. 2B
The following description discloses each displaying stage of a conventional transflective electrowetting display. Referring to FIG. 3A, a conventional transflective electrowetting display 100a includes a substrate 137. A patterned transparent pixel electrode 135 is disposed on the substrate 137. A reflector 136 is interposed between the transparent pixel electrode 135 and the substrate 137, thereby defining a reflective region and a transmission region. A dielectric layer 134 having a hydrophobic surface is disposed on the transparent pixel electrode 135. Patterned hydrophilic bank structures 133 are disposed on the dielectric layer 134, defining a pixel region. An opaque non-polar fluid 132 containing a black dye and transparent polar fluid 131 are disposed in each pixel region. A backlight unit 138 is disposed on the back of the substrate 137. An opposing substrate 140 with a transparent electrode 142 (i.e., common electrode) thereon is disposed opposite the substrate 137. When applied voltage is off, the opaque non-polar fluid 132 is uniformly distributed on the pixel region, whereby a dark state is rendered.
As the applied voltage increases to slightly exceed the threshold voltage, the opaque non-polar fluid 132 converges to expose a portion of the reflective region. At this stage, only a portion of incident light LI is reflected. A main portion of the pixel region is reflective, as shown in FIG. 3B. When the applied voltage is greater than the threshold voltage but less than the saturated voltage, the opaque non-polar fluid 132 is further converged such that the exposed reflective area is greater than the exposed transmission area, as shown in FIG. 3C. When the applied voltage is greater than the saturated voltage, the opaque non-polar fluid 132 completely converges, thus exposing both of the entire reflective and transmission regions, as shown in FIG. 3D.
As the applied voltage increases, pixels of the conventional transflective electrowetting display exposes the reflective region prior to the transmission region, resulting in gray scale variations and gray scale control difficulties under outdoor and indoor ambient environments. Furthermore, the stability and quality of the electrowetting display are affected by gray scale variations.